This invention relates to stencil screen printing machines and sheet feeders therefor.
Stencil screen printing machines and sheet feeders therefor have been known for several decades, see, e.g., U.S. Pat. Nos. 2,578,779; 2,606,492; 2,866,405; 3,029,927 and 3,120,180.
In operating these machines, the individual sheets are sometimes fed to the print cylinder and screen frame by what is called "stream feed" and sometimes by what is called "sheet feed." Stream feed means the delivery to the print cylinder of sheets in overlapped relationship. Sheet feed means the delivery to the print cylinder of sheets in separated relationship, i.e., each spaced from the next. Feeding of the sheets is normally with shifting suction units to a feed table over which recirculating belts pass. When sheets are stream fed, the sheets are fed from the rear of the stock board on which the stock of sheets is placed. That is, the top sheet in the stack is pushed at its rear edge toward the feed table, i.e., to the moving belts to advance the sheets to the print cylinder. When sheets are sheet red, they may be fed by a pusher mounted at the rear of the stock board, i.e., by pushing the rear edge of the sheet, or by a puller mounted at the front of the stock board, i.e., by pulling the front of the sheet. It would be advantageous to have a stencil screen printing machine which could stream feed from the rear, or sheet feed from the front, or sheet feed from the rear, and capable of changeover between these three variations in a quick, easy manner.
Another feature commonly found on stencil screen printing machines is the capacity of the squeegee to be vertically moved between a lowered print position, a slightly raised return, ink flood position, and a high lift position out of the stencil screen frame. In the print position, the squeegee moves over the stencil screen in engagement therewith, to force ink through the screen and onto a sheet of stock held on and moving with the underlying print cylinder. In the return stroke, the flood position is assumed wherein the squeegee is a small fraction of an inch above the screen to be just out of engagement with the screen so as to flood a new layer of ink onto the screen for the next print stroke. High lift of the squeegee is achieved as with a pair of fluid cylinders, gears, or otherwise, which lift the squeegee subassembly completely out of the screen frame so as to allow changes to be made in the squeegee, changes to or removal of the screen frame, cleaning and/or repair of the screen, and the like. When high lift is accomplished by fluid cylinders, these same cylinders have been known to also be used to hold down the squeegee on the stencil screen during the print stroke.
Sometimes a press operator would like to cycle the press through a print stroke to check the print stroke of the apparatus, but without printing during that stroke. The only way that can presently be attempted is to put the squeegee in high lift condition/position and run the press through the forward stroke. However, if this is done, the next stroke cannot be effectively used to print because it takes too long, for the high lifted squeegee to lower onto the screen, so that the press is already part way through the print stroke by the time the squeegee engages the screen. It would be of significant advantage for a stencil screen printer to have a "nonprint" squeegee position allowing a simulated print cycle of a rapidly moving printer, without actual printing taking place, followed by a full print cycle, as well as having the usual high lift position.
Aside from these above noted shortcomings of conventional stencil screen printing machines, there is another shortcoming or trouble area that involves the print cylinder drive. The print cylinder is rotatably mounted to rotationally oscillate in one rotary direction, and then return in the opposite rotary direction, rapidly and repeatedly. Such a print cylinder is typically rotatably oscillated by having a gear on one end of the cylinder engaging an underlying gear rack mounted in a horizontal slide track below the gear. The gear rack can be oscillated linearly while held securely against vertical and lateral movement, assuring constant full engagement with the gear. However, experience has shown that any small object falling into the gear rack, even small metal shavings, can instantly bind up the drive, score the components and jam the fast moving press to stop it, practically instantaneously, with likely concomitant damage. Significant down time, e.g., several days, also results because the printing apparatus must be substantially disassembled by removal of the squeegee, removal of the print cylinder and other adjacent components to get down to the jammed, damaged gear drive mechanism. It would be highly advantageous to have a dependable print cylinder drive using gear and gear rack components but without the known tendency to be jammed and damaged, causing significant down time and repair costs. Moreover, it would be advantageous to have such a print cylinder drive which is removable and replaceable without having to remove the squeegee subassembly or the print cylinder.